You can see the bottom of it underneath the cowl — I think it's visible in an assembled car anyway (I don't have one to check

Here's a pic of up under the passenger side cowl, you can see the triangular bit that is the bottom of the a pillar reinforcement, outlined below in red.
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Here's my 70 that doesn't have it
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Here's a pic of it revealed, my 72.
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and a pic of the same on my 70 that doesn't have the center reinforcement.
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I believe the shape of the cowl, the rear top of the fender, and even the doors changed shape along with this change. I'm hoping to find donor metal, and without knowing when it changed, I don't know what to look for.
Anyone that has a 70 or a 71 could you check yours and let me know?